1. Field of the Invention
The present invention relates to a fuel cell, particularly to cooling of the fuel cell using a separator interposed between unit cells.
2. Description of Related Art
A unit cell of a PEM-type fuel cell is composed of an anode (referred to as a hydrogen electrode since hydrogen gas is generally used as a fuel), an oxidant electrode (hereinafter referred to as a cathode since air including oxygen is similarly used as oxidant), and a solid polymer electrolyte membrane interposed therebetween. Both anode and cathode are composed of catalyst layers, which include catalytic substances, and electrode substrates, which function to support the catalyst layers and to transmit reaction gas and further function as current collectors. Further, on the outside surface of each of the anode and the cathode, is a separator (connector plate) which uniformly supplies an electrode surface with hydrogen or air as reaction gas supplied from the outside of the cell and which is provided with a gas passage (generally structured as a groove with an electrode surface side open) for discharging excess reaction gas from the cell. This separator prevents gas transmission and collects current for supply to the outside. The above-described unit cell and the separator compose one unit of an electric cell.
In an actual fuel cell, a plurality of the unit cells are assembled in series into a stack. In such a fuel cell, it is necessary to keep the solid polymer electrolyte membrane in the unit cell in a fully moist state in order to maintain sufficient power generation efficiency. Generally, the water generated by the electrolytic reaction is insufficient so that a means for supplying humidification water to each unit cell is required. In addition, a substantially proportional quantity of heat to generated power is generated by the electrolytic reaction, so that a cooling means for preventing the fuel cell itself from being overly heated is required.
As a cooling means for the fuel cell, various means have been proposed which also wet the solid polymer electrolyte membrane. For example, in Japanese Patent Laid-open No. Hei 10-247505, air to which water has been previously added is supplied and the water is evaporated within a cooling gas passage for cooling, with circulation of the air that includes the evaporated moisture.
Japanese Patent Laid-open No. Hei 6-338338 shows a hollow portion, separate from the gas passage formed in the separator. Cooling water passed through the hollow portion supplies the air passage with vapor through a porous wall surface.
According to the prior art, it is difficult to maintain both cooling and wetting of the membrane. For example, according to Japanese Patent Laid-open No. Hei 10-247505, with recirculation of the air that includes vapor generated by evaporating liquid water in the cooling gas passage, it is difficult to maintain the temperature of the cooling gas passage in the circulation path. For example, when temperature increases in the air passage after the temperature decreases in the circulation path, the moisture is removed from the electrolyte membrane into the air passage, which makes it difficult to maintain wetting of the membrane.
In Japanese Patent Laid-open No. Hei 6-338338, though the vapor is supplied through a porous wall surface, the supplied vapor is not necessarily sufficient and, in the cooling water passage, cooling is only by sensible heat so that enormous mechanical equipment and energy for circulating the cooling water may be required for complete cooling.